Extensive inter- and intra-cellular amino acid recycling poses difficulties in establishing the contributions of synthesis and degradation to overall change in tissues protein content in the whole tissue or intact organism. This research is a continuation of our studies of a novel method proposed for measuring the true rate of degradation of muscle proteins, myosin actin, in experimental animals. The method is based on rate at which radioactivity of the derived amino acid 3-methylhistidine (prosmethylhistidine) is lost fram muscle protein, following injection of a suitable labeled percursor. Changes in muscle protein degradation rates in response to various physiological and pathological conditions will be examined. Studies will be extended to human subjects. The release of 3-methylhistidine across the forearm and leg muscles will be examined and the effects of insulin administration and exercise on this release will be determined. The urinary excretion of 3-methylhistidine ( and its metabolite) will be measured in human subjects under a variety of physiological and pathological conditions. The rate of muscle protein turnover will be compared in a series of mammals of different body size. BIBLIOGRAPHIC REFERENCES: Haverberg, L.N., Omstedt, P.T., Munro, H.N., and Young, V.R. 1975. Nt-methylhistidine content of mixed proteins in various rat tissues. Biochim. Biophys. Acta 405: 67-71. Haverberg, L.N., Deckelbaum, L., Bilmazes, C., Munro. H.N., and Young, V.R. 1975. Myofibrillar protein turnover and urinary Nt-methylhistidine output. Biochem. J. 152:503-510.